New Life for Old Tires

Of the nearly 300 million tires discarded in the United States each year, more than half end up either as landfill or are burned for fuel in cement kilns and in other industries.

Lehigh Technologies of Tucker, GA, has developed a process for rejuvenating discarded rubber that could open up new recycling opportunities. If the company’s technology catches on, it could carve out a billion-dollar market for high-performance recycled rubber.

Used rubber is hard to recycle because it is vulcanized–hardened and rendered chemically inert–by the addition of sulfur and other compounds to the material’s long molecular chains. Small chunks of used tires can be partially melted and used as filler in asphalt, but devulcanizing rubber involves expensive chemical and thermal processes.

Lehigh Technologies instead shatters rubber into a fine powder using a process that involves freezing old rubber and smashing it to pieces. This starts with tires that have been torn into half-inch chunks using conventional shredding equipment. Lehigh mixes these rubber pieces with liquid nitrogen, cryogenically cooling the rubber to -100°C. The rubber is then fed into a high speed “turbomill” that shatters it into particles no more than 180 microns in size.

Creating such fine powder transforms the rubber from a highly inert filler material to one that can bond with other materials. “We deliver a huge increase in surface area relative to size, and that allows for a much more intimate mixing with other materials,” says Lehigh Technologies CEO Alan Barton.

In 2006, Lehigh Technologies opened its first commercial facility, which has a capacity to produce 100 million pounds of rubber powder and to process four million tires per year. Sales of the company’s products increased by 40 percent last year, but the facility is still operating at less than half capacity. Barton says that his firm has sold recycled rubber to a number of leading tire manufacturers. He estimates that 30 million tires now on the road in the United States are made in part with his company’s recycled rubber, although only about 3 to 7 percent of all the rubber in these tires is their recycled material.

This is largely because Lehigh’s rubber is still technically vulcanized. Carbon atoms in the rubber are still bound to sulfur atoms, and these bonds prevent them from forming covalent bonds with surrounding materials.

The company recently opened an in-house research center that is looking to change the chemical properties of powders it produces, to make their surfaces more reactive. The company has also developed ways to make recycled rubber bind to surrounding materials via noncovalent, intermolecular bonds.

Nearly a third of Lehigh’s annual output also goes to specialty applications, from paints and coatings to injection mold plastics. Lehigh’s PolyDyne and MicroDyne powders can be used to replace as much as 40 percent of the polymers that normally go into plastic.

PolyDyne, the larger and less expensive of Lehigh’s two rubber powders, sells for just under 50 cents a pound; finer grained MicroDyne requires colder temperatures and higher milling speeds, making it significantly more expensive. PolyDyne is half the cost of nonrecycled synthetic rubber, a third of the price of natural rubber, and nearly half the cost of polypropylene, a polymer commonly used in plastic moldings.

This is an area that Lehigh’s investors are particularly interested in.

“Pick whatever plastic product you want to make and it will have specific technical performance requirements,” says Ben Kortlang a partner at venture capitol firm Kleiner Perkins Caufield & Byers, which recently invested in Lehigh Technologies. “Using a blend of PolyDyne and traditional materials, there will typically be a cost savings and, in many cases, a performance improvement. And many of these markets could be very, very large.”

I’m a freelance writer interested in a wide range of topics but focused primarily on energy and the environment. In addition to the pages of MIT Technology Review, you can also find my work in the… MoreNew York Times, Wired, Popular Mechanics, Smithsonian, and New Scientist. You can follow me on Twitter at twitter.com/mckennapr.